I. Technical Field
This invention relates to a method of removing scale from the surfaces of a vertical tube reaction apparatus, including the downcomer and upcomer pipe walls, which contact the aqueous phase waste stream treated by the apparatus. The method comtemplated herein provides for either complete or partial washing of the system.
II. Description of the Background Art
Innovative vertical tube wet oxidation reaction systems are currently being developed to process various organic materials in an acqueous phase. In the wet oxidation process, complex organic materials in an aqueous phase are converted into water, carbon dioxide, dilute organic acids and a small amount of sterile inert ash.
The chemical reaction occurs within a series of vertical concentric tubes which generally extend downwardly into the ground to a depth of from 3,000 feet to 6,000 feet. The fluid to be treated is pumped into one annulus, i.e., the downcomer, formed by adjacent concentric tubes with other reactants for the chemical reaction wherein a fluid head creates pressure and heat is added which facilitates the reaction. The temperature and pressure are greatest in the lower regions of the tubes where the reaction occurs. Following the reaction, the fluid continues its continuous flow upwardly through a second annulus, i.e., the upcomer, where the effluent may be processed further. One example of the chemical reaction is the wet oxidation of municipal wastewater treatment sludges. Oxygen, or air, is added to the influent flow of municipal wastewater treatment sludge which is wet oxidized to form water, carbon dioxide, dilute organic acids and inert ash.
The tubes are preheated and temperature controlled by means of a heat exchange system as disclosed in U.S. Pat. No. 4,272,383 of Dr. McGrew, entitled "Method and Apparatus for Effecting Subsurface, Controlled, Accelerated Chemical Reactions", assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference.
A key to the commercial success of such vertical tube reaction systems is their energy efficiency realized by applying the natural principles of gravity and thermodynamics to create the heat and pressure required to sustain the reaction.
Wet oxidation of the fluid waste stream occurs in the aqueous phase when sufficient oxygen, heat and pressure are present in the system. The wet oxidation reaction is an exothermic reaction which is capable of autogenic operation after the lower portion or reaction zone of the vertical tubes are preheated to the appropriate temperature for oxidation of the waste stream, generally between 300.degree. F. and 600.degree. F.
The efficiency of this system is also seen in the fact that the pumps injecting the fluid waste stream theoretically only need be large enough to overcome the wall friction and any differential head between the influent and effluent tubes. The vertical tubes are designed to provide sufficient residence time of the fluid waste stream in the reaction zone to complete the oxidation reactions. In the preferred arrangement, the influent and effluent flow paths of the fluid waste stream are separated by a single tube which creates an extremely efficient counterflow heat exchange between the influent and effluent wherein the influent draws heat from the effluent as the influent flows toward the reaction zone.
To maintain the extradordinary efficiencies of the system, it is vital that the walls of the tubes remain substantially free of inorganic scale and that no other accumulations or plugging occur. Scale build-up on the walls of the tubes increases the wall friction and reduces the available cross-sectional area through which the fluid waste stream may flow, thereby increasing the load on the pump circulating the fluid waste stream.
Scale build-up on the walls of the vertical tubes also reduces the efficiency of the counterflow heat exchange between the influent and effluent through the walls of the tubes separating the two flows. Similarly, scale accumulations on the wall of the tube adjacent the heat exchange medium reduces the efficiency of preheating the reaction zone.
One problem to which the present invention is directed is the build-up of layers of inorganic scale on the tube surfaces and plugging of the downcomer with organic material. The scale is in part hard anhydrite scale which is comprised of calcium sulfate (CaSO.sub.4). In the environment of the wet oxidation treatment apparatus, severe pyrolysis, hydrolysis and oxidation reactions occur. In this environment, the retrograde solubility of calcium sulfate results in the precipitation of the calcium sulfate and formation of the anhydrite scale on the tube surfaces which are hottest. With calcium sulfate and other substances which are subject to retrograde solubility, as the heat of a solution increases the solubility decreases. Thus, in the reaction zone, which is at a temperature of approximately 500.degree. F. to 600.degree. F., a certain quantity of calcium sulfate will precipitate out of the solution.
The formation of anhydrite hard scale in water distillation devices has long been a problem requiring periodic boil out of distillation devices with deionized water. This approach is not appropriate for use in wet oxidation treatment apparatus due to the degree of scale build-up of substances having retrograde solubility at the extremely high temperatures encountered in the reaction zone comprising portions of the vertical tubes at a depth of more than 3,000 feet.
Mineral acid-type scale removers currently recommended for machine cleaning are relatively ineffective on anhydrite or silicate scales and used alone do not provide the necessary de-scaling action. Alkali cleaners are ineffective on mineral carbonates which also form a component of the scale build-up in the wet oxidation treatment apparatus. Other commercially available descalers have been tried to eliminate the scale build-up in the wet oxidation treatment apparatus, however they are either subject to degradation in the high temperatures of the reaction zone or they are excessively corrosive, which would reduce the useful life of the wet oxidation treatment apparatus.
Another problem to which the present invention is the organic and inorganic scaling and resulting plugging of the upper section of the reaction vessel in the downcomer. The formation of plugs in this area greatly increases the pressure drop across the reaction vessel, thereby requiring cleaning before the process can continue effectively.